This invention relates to thickened aqueous solutions, and more particularly to a method of thickening low-pH, aqueous, surfactant-containing compositions, and the compositions so thickened.
It is desirable, for various reasons, to thicken, or increase the viscosity of, aqueous solutions. Additives for thickening are widely known, and include nonionic thickeners, exemplified by natural gums such as guar and locust bean extract, starches and cellulose and their derivatives, such as hydroxyalkyl celluloses and carboxyalkyl celluloses. They also include ionic thickeners, exemplified by certain polyelectrolyte resins such as polymers of acrylic acid and methacrylic acid, and copolymers containing hydrophobic groups based upon long, fatty-acid constituents and hydrophilic linkages based upon poly(oxyethylene) chains.
The nonionic thickeners are relatively insensitive to pH, but they are also relatively expensive, require special processing techniques for incorporation into aqueous compositions, must be used in comparatively high concentration, and tend to exhibit viscosity loss on storage as a result of hydrolysis. The ionic thickeners are therefore of greater interest, since they overcome many of these deficiencies. Especially of interest are the alkali-soluble emulsion polymer thickeners which are supplied as acidic emulsions of relatively low viscosity, and are neutralized at least partially to the salt form either prior to, simultaneously with, or subsequent to incorporation into the aqueous compositions. The neutralization is accompanied by dissolution of the thickener emulsion, and by a marked increase in the thickener viscosity. Especially in the case of detergents and other surfactant-based compositions, the composition itself is neutral or alkaline, so this neutralization of the thickener emulsion presents no problem.
A need exists, however, for thickening acidic surfactant compositions. While by no means the only such acidic surfactant compositions, examples of these are the "natural-pH" shampoo compositions, having a pH from about 5.5 to about 6.5, a range said to include the pH of healthy human hair and scalp. As the alkali-soluble emulsion polymer thickeners are insoluble in acidic solutions, and only develop high viscosities upon neutralization to a pH above 6.5-7.0, they are not used in the acidic surfactant compositions. Rather, the nonionic thickeners are used because of their stability at low pH, despite the above-listed disadvantages.